The Science of Fish and Courtship: 28th April

Male Ospreys walk a fine line to ensure that they breed successfully. On the one hand they feel a need to guard their mate on the nest to make sure that she does not breed with other males and on the other hand they need to make sure she is well fed and so does not go off fishing for herself or be tempted to visit other males for food.

At the start of the courtship it is in his best interests to shower his partner with fish. This phenomenon is well observed in Ospreys and is known as “courtship feeding”. 

If we take a look at fish deliveries to the nest this year so far we can see this clearly at work:

Courtship Feeding 2014
Courtship Feeding 2014

Before laying her eggs our female osprey spent almost all her time at the nest and was brought fish several times a day by our male. The rate at which he delivered fish was highest early in the season and he started with a bonanza of three fish. This number began to decline through the pre-laying period from two to one fish a day. Our female is however pretty vocal when she is hungry and will nag persistently until a fish eventually appears.

  Our male is maintaining an average of 1.6 fish per day and this falls within a range that has been seen in other Scottish birds. He may be enjoying the incubation lull because once the eggs have hatched he will have to drastically up his daily catch and this will only have to increase as the chicks grow.

So far there have been 44 fish deliveries by our male to the female on the nest within the 28 days they have been a couple. Of these, 21 have been identified easily.  We are keeping all the footage though and will keep working to identify the rest of the mystery fish. 

The table below shows that Brown Trout are forming the backbone of our birds’ diet, followed by Pike.

fish deliveries to late April

The reason for the partiality for these two species can be explained by the availability of fish in the local vicinity and by the seasonal habits of these fish. Ospreys are highly specialised fish hunters and will eat fish they can catch on the upper 1 meter layer of the water. Pike spawn in spring, mainly in April and these adults may be vulnerable to Osprey at this time as they spawn in shallow water.

Trout feed on aquatic invertebrates, on the immature insects with aquatic developmental stages such as dragonfly and on terrestrial insects trapped in the water. A study of two Perthshire streams showed that trout fed mainly on emerging and terrestrial insects from May to October and these may be particularly available for Ospreys when rising for surface food.

Fish deliveries at Lowes also tend to show a temporal pattern across the day with deliveries taking place early in the morning and then again in the early evening. The graph below shows this general tendency.

In fact as daylight hours are increasing the early morning deliveries appear to be getting earlier and earlier as you would expect.

Timing of Fish Deliveries 2014

Timing of Fish Deliveries 2014

By Kirstin Mair, Species Protection Officer.

Preface

Male Ospreys walk a fine line to ensure that they breed successfully. On the one hand they feel a need to guard their mate on the nest to make sure …

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